QUALITY SCANNING:

Creating high quality scans for raster to vector conversion

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Resolution

It is not true that "the higher the scanner resolution, the better the vectorization results". In fact, a high resolution scan can sometimes give you worse raster to vector results than a low resolution scan!

That said, you should be aware that while you can decrease the resolution of an image after scanning you cannot increase it. Increasing resolution after scanning will not regain any lost detail. It will simply exacerbate "steps" in the image that will decrease the quality of any raster to vector conversion.

Therefore, it is better to err on the side of too high resolution rather than too low resolution when scanning. If you find your scan resolution is too high you can always decrease it after the fact.

For most drawings, a scan resolution of 200 to 400 dpi is optimal. Here are some pointers to look out for:

When resolution is too low, breaks often appear in the drawing and text becomes illegible.
When resolution is too low, curves often look stepped instead of smooth.

Stepped

Smooth
When resolution is too low, text characters and entities that are close together become joined.

Joined

Separated

(Note that the separation of close together entities is dependent on selecting an appropriate threshold as well as on selecting an appropriate resolution.)